1. Field of the Invention
The present invention relates to an optical signal receiving circuit and an optical signal receiving semiconductor device used for a digital signal photocoupler, an optical digital data link, and the like.
2. Description of the Related Art
FIG. 7 is a block diagram showing the circuit configuration of a related digital optical signal receiving circuit 5, and FIG. 8A and FIG. 8B are diagrams showing voltage waveforms in various nodes of the optical signal receiving circuit 5.
As shown in FIG. 7, the related optical signal receiving circuit 5 includes photodiodes 10 and 12, transimpedance amplifiers 14 and 16, a differential amplifier 20, a comparator 22, an output circuit 24, and a light shield 30.
An optical signal is inputted to the photodiode 10, and the photodiode 10 generates a current signal in response to the optical signal. The current signal is converted into a voltage signal in the transimpedance amplifier 14. An example of this voltage signal is S1 in FIG. 8A. This voltage signal S1 is then inputted to the differential amplifier 20.
On the other hand, no optical signal is inputted to the dummy photodiode 12 since the light shield 30 is provided for the dummy photodiode 12, and hence the dummy photodiode 12 generates only a current signal based on noise and the like. This current signal based on noise and the like can be assumed to be generated in the same manner as in the photodiode 10. The current signal based on noise and the like which is generated in the photodiode 12 is converted into a voltage signal in the dummy transimpedance amplifier 16. This voltage signal is raised by a voltage V1 by a voltage source V1 and inputted to the differential amplifier 20. An example of this voltage signal is S2 in FIG. 8A. Incidentally, the reason why an offset of the voltage V1 is provided is that the operation of the comparator 22 is stabilized by allowing the voltage signal S2 to have a higher voltage when the voltage signal S1 being an output of the transimpedance amplifier 14 is nothing.
The differential amplifier 20 amplifies a difference between these voltage signals S1 and S2 and outputs a equilibrium signal S3, and concurrently outputs a equilibrium signal S4 obtained by inverting the equilibrium signal S3. Respective examples of the equilibrium signals S3 and S4 are shown in FIG. 8B. These equilibrium signals S3 and S4 are inputted to the comparator 22. The equilibrium signals S3 and S4 are outputted to the output circuit 24 after their waveforms are adjusted in the comparator 22. The output circuit 24 outputs a digital signal based on the equilibrium signals S3 and S4.
The aforementioned optical signal receiving circuit 5, however, has a problem that when the operating region of the differential amplifier 20 is in a clip region, clip voltage is outputted to the equilibrium signals S3 and S4, and hence an accurate digital signal is not obtained. Namely, if the equilibrium signal S3 is taken as an example, when the differential amplifier 20 operates in a non-clip region as shown in FIG. 9, the equilibrium signal S3 can draw a correct waveform according to a photocurrent as shown by a full line. When the differential amplifier 20 operates in the clip region, however, the equilibrium signal S3 is clipped with the clip voltage of the differential amplifier 20 as shown by a dotted line, and hence it cannot draw a correct waveform according to the photocurrent. There arises a problem that if the digital signal is generated by use of such equilibrium signals S3 and S4, the pulse width of the digital signal increases.
Moreover, in the photodiode 10 and the transimpedance amplifier 14 which convert the optical signal into the current signal, a tail 40 such as shown in FIG. 8A is sometimes caused by a diffusion current in the photodiode or the like when the optical signal is about to disappear. If the tail 40 occurs in the voltage signal S1, the tail 40 is amplified by the differential amplifier 20, which causes a problem that the pulse width of the outputted digital signal is increased or a pulse combines with the next pulse. Namely, as shown in FIG. 8A, if the tail 40 occurs in the voltage signal S1, the cross point between the equilibrium signal S3 and the equilibrium signal S4 is shifted, and hence a distortion 42 occurs in the pulse width of the outputted digital signal.